It is well known in photographic science that the sensitometric performance of silver halide microcrystals can be altered by so-called "chemical sensitizers." Operationally, the function of this special class of photographic addenda is to decrease the number of photons required to create a development center (i.e., increase photographic speed), independent of wavelength.
For most applications, chemical sensitizers are compounds which incorporate sulfur and/or gold into silver halides during emulsion making and/or finishing. Until relatively recently gold sensitization was frequently combined with sulfur sensitization through the use of formulations often based on aurous dithiosulfate [e.g., Na.sub.3 Au(S.sub.2 O.sub.3).sub.2 2 H.sub.2 O]. However, during the past decade the discovery and synthesis of various "gold-only" Au(I) sensitizers, such as described in U.S. Pat. Nos. 5,049,485; 5,700,631; and 5,620,841, has led to increased flexibility of chemical sensitization and increased control of components introduced into photographic makes.
Promising gold-only sensitizers are Au(I) complexes with relatively labile ligands, but without active sensitizing sulfur. Historically, the syntheses of suitable gold-only sensitizers has been difficult for a variety of reasons. Frequently, the materials compounded are either too insoluble to make an aqueous "doctor" solution or their aqueous solutions are too unstable and degrade rapidly upon keeping. Other materials, for example, some Au(I) complexes, i.e., KAu(CN).sub.2 and some Au(I) phosphino complexes, have good solution stabilities, but are so stable that emulsions prepared with them do not exhibit speed enhancement.
Because of the complexity of photographic chemistry in general and the minute quantities of silver and/or gold necessary to create a development center, little is known of the incorporated gold species which is formed during chemical sensitization. One of the explanations proposed for photographic enhancement by gold sensitization is based on the observation that silver is plated onto a gold electrode surface at a less negative potential than is silver deposited onto a silver electrode. In the electrochemical literature this phenomenon is known as "underpotential deposition" (or UPD) and has been observed for a variety of electrode metals and metal-ion combinations (Bard and Faulkner, "Electrochemical Methods," John Wiley and Sons, New York (1980), p. 308).
In the chemical sensitization mechanism, UPD of silver onto Au(0) which has been deposited onto the surface of the silver halide microcrystal would theoretically accelerate development and/or render otherwise subdevelopable latent images developable. Either route would produce an increase in photographic speed (Hillson and Adam, "On Latent Images of Gold and Silver, " J. Photogr. Sci, 23, 104 (1975)).
In the late 1970's, the chemical sensitization of photographic AgBr-polyvinyl alcohol (PVA) emulsions with Au(I) complexes of sulfur or phosphorous-containing ligands was investigated (Suss, Schroter, Reinhold, Zwanziger, and Hoyer, "Gold Sensitization of Silver Bromide Layers in Polyvinyl Alcohol on Glass," J. Signal AM 5 (1977) and Hartung, Schroter, Reinhold, Zwanziger, Dietzsch, and Hoyer, "Chemical Sensitization of Photographic Silver Bromide-Polyvinyl Alcohol Emulsions with Gold(I) Complexes of Sulfur or Phosphorus-Containing Ligands," J. Signal AM 58 (1980)). In the earlier of these two papers, literature values of standard reduction potentials were shown to correlate, within certain limits, with a given complex's ability to chemically sensitize AgBr in a PVA matrix. In the second paper, polarographic half-wave potentials obtained in acetonitrile were used to experimentally verify the relationship between the ease of electrochemical reduction of a complex and its tendency to chemically sensitize.
While concentrating on the mechanistic relationship between electrochemical reduction potentials and gold (I) chemical sensitization, neither of these aforementioned studies investigated water-soluble gold (I) complexes nor related the stability of the complex in aqueous solution to photographic usefulness. The inventors herein have discovered a method which permits the establishment of a "utility window" of electrochemical potentials which permits the complex to be screened for practical manufacturing suitability.
Such a method for easily and accurately screening gold chemical sensitizers is needed to avoid time consuming and costly experimentation in evaluation such compounds.